High-precision laser machining applications can impose stringent performance requirements on laser systems. Laser drilling of the numerous small via-holes commonly incorporated in multi-layer printed circuit boards to provide inter-layer connections exemplifies such an application. Accurate placement and consistent dimensions of the laser-drilled holes must be maintained throughout a production cycle that may include many thousands or even millions of nominally identical holes. Consequently, to be considered suitable for via-drilling, a laser system should exhibit near-constant time-averaged output power and steady beam pointing during extended periods of operation. A diode-pumped solid-state (DPSS) laser can be especially attractive in this regard, but may still be problematic.
Effective laser machining of the materials commonly employed in the manufacture of printed circuit boards generally favors the use of short optical wavelengths, typically in the ultraviolet (UV) or deep-ultraviolet (DUV) portions of the electromagnetic spectrum. UV and DUV wavelengths tend to be more strongly absorbed than longer wavelengths, supporting rapid drilling rates. Short optical wavelengths are also advantageous when small focal spots are required, important for obtaining superior precision and high energy density. However, since DPSS lasers generally include gain media that most efficiently generate infrared (IR) rather than shorter wavelengths of light, a variety of techniques have been developed and widely adopted to efficiently convert DPSS laser outputs from the IR to the visible and even to UV or DUV wavelengths.
A process termed second harmonic generation (SHG) is routinely used to double the frequency and halve the wavelength of near-IR fundamental radiation having a wavelength near 1000 nanometers (nm) to produce visible light having a wavelength near 500 nm. In this context, SHG commonly involves propagating an IR output beam from a DPSS laser resonator through an appropriate optically nonlinear crystal, for example a crystal of lithium triborate (LBO). When such a crystal is properly tuned by establishing an appropriate orientation and temperature, visible light is generated and exits the crystal, usually accompanied by some residual fundamental-wavelength light. The efficiency of converting power from the fundamental wavelength to the desired frequency-converted wavelength (conversion efficiency) is defined by the ratio of the net power transferred to the frequency-converted output divided by the power contained in the fundamental-wavelength source beam. IR-to-visible conversion efficiencies exceeding 50% in LBO are readily demonstrated.
To extend this conversion process to an even shorter wavelength, a second optically nonlinear crystal can be configured to mix the visible SHG output from a first crystal with the residual IR-wavelength light to generate the third harmonic of the fundamental. For a fundamental wavelength of 1064 nm, a preferred operating wavelength for DPSS lasers including neodymium-doped yttrium aluminum garnet (Nd:YAG) or neodymium-doped yttrium orthovanadate (Nd:YVO4) gain media, this third harmonic generation (THG) process yields a UV wavelength of 355 nm.
Alternatively, a second frequency-doubling stage may instead be arranged to act upon the SHG output alone to generate yet another and even shorter wavelength. This process is termed fourth harmonic generation (FHG) since the second-stage frequency-converted output has an optical frequency four times greater than that of the fundamental IR radiation. For the 1064 nm fundamental wavelength case cited above, FHG yields a frequency-converted DUV wavelength of 266 nm.
As noted above, optimization of a frequency converter generally involves establishing an appropriate wavelength-dependent crystal orientation and operating temperature. Operating parameters that optimize conversion efficiency for a particular frequency converter may be determined during a preliminary characterization or calibration phase of system operation. Optimal operating parameters might be determined once and thereafter be left unchanged in anticipation of retaining the demonstrated performance without subsequent intervention by a system operator.
Such an optimistic approach typically encounters problems. Over time, the frequency-converted output power of a DPSS laser, even given constant input power, tends to degrade as optical components age and accumulate damage. In addition, as the laser system power level or operating duty factor change, re-tuning of the frequency converter often becomes necessary and some method of monitoring conversion efficiency becomes advisable.
One method to compensate for deteriorating frequency-converted output power involves monitoring the frequency-converted output power level and increasing pump power as needed to boost the output while also monitoring the fundamental power level to allow determination of the conversion efficiency. Adjustments may then be made to the frequency converter to maintain or recover the desired efficiency. This approach does not always give satisfactory results.
The time-averaged power and perhaps even the position of the frequency-converted output may change so much over time that no reasonable adjustment of the diode drive current alone can recover the desired operating condition. Further, when fundamental and frequency-converted beams are sampled and detected separately, components exposed to the different beams may degrade at different rates. Such differential aging may bias the assessment of conversion efficiency. In addition, verifying that peak conversion efficiency is being maintained, when the output power may be slowly varying due to changes in the laser resonator, necessitates detuning the frequency converter away from an optimal condition, checking for a corresponding roll-off in performance, then re-tuning back to the optimal value. Another concern is that in a laser system exhibiting substantial short-term power fluctuations, such as those associated with intermittent pulsed operation, power monitoring may give erratic or misleading results due to transient thermal effects.
It would therefore be desirable to develop an alternative method of optimizing the conversion efficiency of a laser frequency converter. Ideally such an alternative method would be capable of establishing and verifying an optimal operating condition without requiring detuning, and would be less sensitive to intermittent average power fluctuations or power cycling. Such a method could then be used to improve the long-term power stability and reliability of a frequency-converted laser system.